Breast tumors expressing estrogen receptor alpha (ER) but not HER2 (ER+/HER2-) account for the majority of recurrences and deaths from breast cancer. In patients with early-stage disease, anti-estrogen therapies that suppress ER activity prevent cancer recurrence, but ~33% of patients (~300,000 women diagnosed each year) eventually develop recurrent disease. Advanced/metastatic breast cancer is managed with further anti- estrogen therapies, targeted therapies, and DNA-damaging chemotherapies. Nearly all metastatic breast cancers eventually become completely refractory to these therapies. Prior to the approval of tamoxifen, estrogens were frequently used for the treatment of breast cancer. This may seem counterintuitive since we now rely on anti-estrogens for disease management, but response rates to estrogens are similar to those of anti-estrogens in the setting of advanced disease. Approximately 1/3 of anti-estrogen-resistant breast cancers respond to estrogen therapy, translating into ~100,000 new patients each year who could benefit. Similarly, some cancers respond to withdrawal of anti-estrogen therapy, which may be caused by ER reactivation. Breast tumor responses to estrogen therapies and anti-estrogen withdrawal have been observed for >70 years, but the lack of A) understanding of therapeutic mechanism(s), and B) criteria to identify patients likely to benefit have hindered clinical use. To legitimize this inexpensive, widely accessible, time-tested, relatively safe and tolerable treatment option, and to provide a precision medicine basis to limit its use to patients with cancers likely to respond, the following critical issues need to be addressed: 1) understanding the mechanism(s) underlying sensitivity of anti-estrogen-resistant breast cancers to estrogen therapy and anti-estrogen withdrawal; 2) identifying tumor markers that predict benefit from ER reactivation therapy; 3) identifying strategies to enhance response; 4) understanding the dynamics of therapeutic response/resistance. We hypothesize that during adaptation to anti-estrogens and estrogen deprivation, ER+ breast cancer cells acquire molecular changes that render estrogen-dependent ER reactivation proteotoxic and deleterious. We will test this hypothesis through the following Specific Aims: 1) Determine whether a finite window of ER transcriptional activation promotes growth of breast cancer cells, and how this window shifts with acquisition of anti-estrogen resistance; 2) Determine how ER reactivation elicits proteotoxic stress-dependent cell death; 3) Determine the optimal dose, duration, and mechanisms of escape from 17b-estradiol therapy in anti-estrogen-resistant breast tumors; 4) Identify baseline and pharmacodynamic biomarkers that predict response to 17b-estradiol therapy in patients with anti-estrogen-resistant breast cancer.